1. Field of the Invention
The present invention relates to a radio control station, a radio terminal, a base station, a communication system, and a communication method.
2. Description of the Related Art
Conventionally, one radio terminal generally received radio communication services through one radio communication system. However, advances of radio techniques have made it possible to provide plural transmitters-receivers in one radio terminal. As a result, radio terminals have become available, which allow the use of plural radio communication systems within the borders such as PHS (Personal Handyphone system) and PDC (Personal Digital Cellular Telecommunication System).
A radio terminal 340 of this type consists of plural communication units 342a to 342c which are transmitters-receivers adapted to respective radio communication systems, as shown in FIG. 1. Each of the plural communication units 342a to 342c independently transmits and receives control signals using a control channel such as a pilot channel of the respective radio communication system to connect a communication path to the respective communication system.
For example, there is determined which of the communication units 342a to 342c is used for the communication depending on system selection information that is input from the outside. For example, an input unit 341 receives system selection information, which is input from the outside and is related to a radio communication system selected by the user of the radio terminal 340. Based on the input system selection information, an input unit 341 instructs one of the communication units 342a to 342c that corresponds to the selected radio communication system to perform communication.
Conventionally, as shown in FIG. 2, when the radio terminal 340 is powered on and activated (S301), it retrieves and receives a pilot channel of each radio communication system to retrieve radio communication systems available for communication and to thereby select a radio communication system to be used for communication (S302). The radio terminal 340 can select a radio communication system by receiving the input of system selection information that is selected by the user of the radio terminal 340.
Alternatively, the last radio communication system through which the radio terminal 340 communicated may automatically be selected. The radio terminal 340 sets the transmission-reception frequency at the frequency of the pilot channel of the selected radio communication system (S303). Then, the radio terminal 340 transmits and receives control signals using the pilot channel to connect a communication path to the radio communication system and to start communication (S304).
Another technique for retrieving systems usable for communication from among plural radio communication systems is disclosed in Japanese Patent Laid-Open No. 2003-9239. According to the technique, a base station transmits a control channel added with control information relating to control channels over which control signals are broadcasted by the other mobile communication systems different from the mobile communication system to which the control channel itself belongs. A radio terminal determines a control channel to receive based on the control information thus provided.
However, according to the method in which a radio terminal retrieves and receives the control channel of each radio communication system to retrieve radio communication systems available for communication, there will be an increase in the number of radio communication systems that the radio terminal can use increases and in the number of control channel frequency bands that must be received, which will result in an increase in the time required for retrieving radio communication systems available for communication. Such an increase in retrieving time results in an increase in power consumption of the radio terminal, which is not preferable from the viewpoint of the duration of the battery of the radio terminal.
According to the method in which a transmitted control channel is added with control information relating to control channels over which control signals are broadcasted by mobile communication system other than the mobile communication system to which the control channel belongs, every radio communication system is required to transmit control information relating to control channels of other communication systems in addition to control information relating to the control channel of itself. This results in an increase in a control load of every radio communication system and in an increase in a control load of a communication system as a whole including plural radio communication systems. Further, each time a new radio communication system is introduced, control information relating to the control channel of the new radio communication system must be added to the control channels transmitted by all radio communication systems. Thus, the introduction of a new radio communication system involves a complicated process.